Vehicle transmissions, particularly automatic vehicle transmissions include both mechanical and hydraulic subsystems. The hydraulic subsystem actuates clutches in order to change the transmission gear speed ratio based on a controller signal.
Included in the hydraulic subsystem are accumulators. Accumulators are used to dampen the harshness of a shift in a transmission by slowing or accumulating the pressure rise in the application circuit of a clutch pack or band. In other words, the accumulator acts as a modifier that affects transmission shift timing and quality. Accumulators absorb the initial shock of pressure changes in the hydraulic circuit. When a clutch or band's piston or servo first strokes, there is low pressure in the circuit. As soon as the piston or servo starts to apply, pressure increases rapidly. Without a shift modifying device like an accumulator, sudden and harsh shifting is likely to occur.
Accumulators are designed to “cushion” or “dampen” a shift by slowing down the apply oil to a clutch pack or a band. Accumulators may overcome spring force, or hydraulic fluid during a shift event to provide this “cushion” or “dampening” effect.
In typical prior art accumulators, the accumulator is made of aluminum and is guided through its stroke by a steel pin through the center of the accumulator piston. The piston is sealed and supported on the outside diameter by a single lathe cut rubber type seal, or a single polytetrafluroethylene (PTFE) sealing ring. Often times, accumulators can leak due to such things as a worn seal, worn bore or worn centering pin. Further, a single seal provides inadequate lateral support that allows the piston to rock in the bore on apply and release. This continual rocking creates wear to the pin bore and/or seal, and, in extreme cases, damages the accumulator bore. This wear creates a leak in the accumulator hydraulic circuit and can contribute to slipping of the brake band, and or the slipping or burning of the clutch pack.
Thus, there is a need for an accumulator piston design that does not result in wear or damage to the accumulator bore or pin bore. The present disclosure addresses this need.